The present invention relates to acicular ferromagnetic alloy particles containing Si, Cr, Ni and P and optionally containing Mg impregnated with phosphorus compound and silicon compound and to the process for producing the same. More in detail, the present invention relates to acicular ferromagnetic alloy particles, containing Si, Cr, Ni and P impregnated with phosphorus compound and silicon compound and also acicular ferromagnetic alloy particles for magnetic recording, containing Si, Cr, Ni, P and Mg impregnated with phosphorus compound and silicon compound, and to a process for producing acicular ferromagnetic alloy particles containing Si, Cr, Ni and P impregnated with phosphorus compound and silicon compound, comprising the steps of,
oxidizing Fe(OH).sub.2 in an aqueous suspension by blowing an oxygen-containing gas into said aqueous suspension of Fe(OH).sub.2 of pH of higher than 11 obtained by reacting an aqueous solution of a ferrous salt with an aqueous solution of an alkali metal hydroxide thereby obtaining acicular particles of alpha-FeOOH containing Si, Cr and Ni, a water-soluble salt of silicic acid being added into said aqueous solution of an alkali metal hydroxide or into said aqueous suspension of Fe(OH).sub.2 before blowing said oxygen-containing gas in an amount of 0.1 to 1.7 atomic % (calculated as Si) to Fe of said ferrous salt, and a water-soluble chromium salt and a water-soluble nickel salt being added into said aqueous solution of the ferrous salt, said aqueous solution of the alkali metal hydroxide, said aqueous suspension of Fe(OH).sub.2 before blowing the oxygen-containing gas thereinto, or said aqueous suspension of Fe(OH).sub.2 during oxidizing by blowing the oxygen-containing gas thereinto, in the respective amount of 0.1 to 5.0 atomic % (calculated as Cr) and 0 1 to 7.0 atomic % (calculated as Ni) to Fe of said ferrous salt,
after collecting said acicular particles of alpha-FeOOH containing Si, Cr and Ni from the mother liquor and suspending the thus obtained particle in water, adding 0.1 to 2% by weight of a salt of phosphoric acid (calculated as PO.sub.3) to said acicular particles of alpha-FeOOH containing Si, Cr and Ni in the thus formed aqueous suspension of pH of higher than 8,
further adding 0.1 to 7.0% by weight of a water-soluble silicate (calculated as SiO.sub.2) thereinto and adjusting the pH of the thus prepared aqueous suspension to 3 to 7, thereby obtaining the acicular particles of alpha-FeOOH containing Si, Cr and Ni coated with a phosphorus compound and a silicon compound
after collecting said acicular particles by filtering and drying said acicular particles, subjecting the thus dried acicular particles of alpha-FeOOH containing Si, Cr and Ni coated with said phosphorus compound and said silicon compound to thermal treatment in a non-reducing atmosphere, thereby obtaining the acicular particles of alpha-Fe.sub.2 O.sub.3 containing Si, Cr and Ni impregnated with said phosphorus compound and said silicon compound,
subjecting the thus obtained acicular particles of alpha-Fe.sub.2 O.sub.3 containing Si, Cr and Ni impregnated with said phosphorus compound and said silicon compound to thermal reduction in a reducing gas thereby obtaining acicular ferromagnetic alloy particles containing Si, Cr, Ni and P impregnated with phosphorus compound and silicon compound, and
a process for producing acicular ferromagnetic alloy particles containing Si, Cr, Ni, Mg and P impregnated with phosphorus compound and silicon compound, comprising the steps of,
oxidizing Fe(OH).sub.2 in an aqueous suspension by blowing an oxygen-containing gas into said aqueous suspension of Fe(OH).sub.2 of pH of higher than 11 obtained by reacting an aqueous solution of a ferrous salt with an aqueous solution of an alkali metal hydroxide thereby obtaining acicular particles of alpha-FeOOH containing Si, Cr, Ni and Mg, a water-soluble silicate being added into said aqueous solution of an alkali metal hydroxide or into said aqueous suspension of Fe(OH).sub.2 before blowing said oxygen-containing gas in an amount of 0.1 to 1.7 atomic % (calculated as Si) to Fe of said ferrous salt a water-soluble chromium salt and a water-soluble nickel salt being added into said aqueous solution of the ferrous salt, said aqueous solution of the alkali metal hydroxide, said aqueous suspension of Fe(OH).sub.2 before blowing the oxygen-containing gas thereinto, or said aqueous suspension of Fe(OH).sub.2 during oxidizing by blowing the oxygen-containing gas thereinto, in the respective amount of 0.1 to 5.0 atomic % (calculated as Cr) and 0.1 to 7.0 atomic % (calculated as Ni) into Fe of said ferrous salt, and a water-soluble magnesium salt being added to said aqueous solution of said ferrous salt, said aqueous solution of said alkali metal hydroxide, said aqueous suspension of Fe(OH).sub.2 before blowing said oxygen containing gas therein, or said aqueous suspension of Fe(OH).sub.2 during blowing said oxygen-containing gas thereinto in an amount of 0.1 to 15.0 atomic % as Mg to Fe corresponding to said ferrous salt in said aqueous suspension of Fe(OH).sub.2, thereby oxidizing said ferrous salt into acicular particles of alpha-FeOOH containing Si, Cr, Ni and Mg,
after collecting said acicular particles of alpha-FeOOH containing Si, Cr, Ni and Mg from the mother liquor and suspending the thus obtained particle in water, adding 0.1 to 2% by weight of a salt of phosphoric acid (calculated as PO.sub.3) to said acicular particles of alpha-FeOOH containing Si, Cr, Ni and Mg in the thus formed aqueous suspension of pH of higher than 8,
further adding 0.1 to 7.0% by weight of a water-soluble silicate (calculated as SiO.sub.2) thereinto, and adjusting the pH of the thus prepared aqueous suspension to 3 to 7, thereby obtaining the acicular particles of alpha-FeOOH containing Si, Cr, Ni and Mg coated with a phosphorus compound and a silicon compound,
after collecting said acicular particles by filtering and drying said acicular particles, subjecting the thus dried acicular particles of alpha-FeOOH containing Si, Cr, Ni and Mg coated with said phosphorus compound and said silicon compound to thermal treatment in a non-reducing atmosphere thereby obtaining the acicular particles of alpha-Fe.sub.2 O.sub.3 containing Si, Cr, Ni and Mg impregnated with said phosphorus compound and said silicon compound,
subjecting the thus obtained acicular particles of alpha-Fe.sub.2 O.sub.3 containing Si, Cr, Ni and Mg impregnated with said phosphorus compound and said silicon compound to thermal reduction in a reducing gas thereby obtaining acicular ferromagnetic alloy particles containing Si, Cr, Ni, Mg and P impregnated with phosphorus compound and silicon compound.
The acicular ferromagnetic alloy particles containing Si, Cr, Ni and P and optionally containing Mg impregnated with phosphorus compound and silicon compound of the present invention is most suitable as a magnetic material for magnetic recording in audio and video, particularly as a magnetic material for video, not being contaminated by any dendrites and not having any entwining particles each other, thus resulting in large in apparent density, large in specific surface area as minute particles, high in crystallinity of the surface and of the inner part of particles, substantially high in density, high in coercive force and large in saturation magnetization thereof.
In recent years, the longer-time recording, the miniaturization and the weight-saving of the reproducing apparatus for magnetic recording for audio and video have been remarkably intensified, particularly with the striking popularization of VTR in these days. Accordingly, the development of VTR has been vigorously carried out with the objects of the longer-time recording, the miniaturization and the weight-saving thereof. On the other hand, the demand for the magnetic tape, i.e., the magnetic recording media, with higher performance and higher recording density has been intensified.
Namely, the high quality of the picture, the high output capacity, particularly the improvement of frequency characteristics and the reduction of noise level are required to the magnetic recording media. For matching these requirements, it is necessary to improve the residual magnetic flux density(Br), the coercive force(Hc), dispersibility, the packing property, the smoothness of the surface of the tape and the signal to noise ratio S/N.
These specific properties of magnetic recording media are in a close relationship with the magnetic material for use in the magnetic recording media, for instance, as are seen in the following description on pages 83 to 84 of an article entitled with "The Recent Progress of Magnetic Tapes for Video and Audio", pages 82 to 105 published by NIKKEI-Electronics (1976) on May 3, 1976.
"The major specific properties which represent the quality of the picture of the video-tape recorder are (1) the video signal to noise ratio S/N, (2) chroma noise and (3) the frequency characteristics of video signal . . . , these specific properties representing the picture are decided by the electro-magnetic tranforming characteristics and the electro-magnetic transforming characteristics is interrelated to the magnetic properties of the tape. Furthermore, a larger part of the magnetic properties of the tape is decided by the magnetic material constructing the tape."
As is described above, the various specificities of the quality of the picture reproduced from magnetic recording media are in close relationship with the magnetic material used for preparing the media, and accordingly, the improvement of the characteristics of the magnetic material is strongly demanded.
The relationship between the various characteristics of the magnetic recording media and the characteristics of the magnetic materials used for preparing the media are described in detail as follows.
In order to obtain a high quality in the picture reproduced from magnetic recording media used in video, as are clearly seen in the afore-mentioned description in "NIKKEI-Electronics", the improvement of (1) video signal to noise ratio S/N, (2) chroma noise and (3) video frequency characteristics.
In order to improve the video signal to noise ratio S/N, it is important to micronize the magnetic particles and to improve the dispersibility thereof in the vehicle, the orientation and packing thereof in the coating medium and the smoothness of the surface of the magnetic recording medium. These facts are clearly recognizable also from the following description in the afore-cited "NIKKEI-Electronics" on page 85.
"As the physical properties of the tape, which are in connection with the signal to noise ratio S/N(CN ratio) of the luminance signal, the average number of the particles per unit volume the state of dispersion thereof(dispersibility thereof) and the smoothness of the surface of the tape are mentioned. In the case where the smoothness of the surface and the dispersibility of the particle are constant, the signal to noise ratio S/N becomes better in proportion to the square root of the average number of the perticle per unit volume and accordingly, the magnetic particle is the more favorable as the volume thereof is the smaller and the degree of packing thereof is the larger. In addition, as a method for improving the video signal of noise ratio S/N, it is an important method to reduce the noise level attributable to the magnetic recording media, and for reducing the noise level, it has been known that the micronization of the particle size of the acicular ferromagnetic alloy particles which are the magnetic material for use in preparing the medium for magnetic recording, as are seen in the afore-cited description."
As a general means for indicating the size of the magnetic particle, the value of the specific surface area of the particle is frequently used, and it has also been generally known that the noise level attributable to the magnetic recording media becomes smaller as the specific surface area becomes larger. This phenomenon is shown, for instance, in the Technical Research Report of DENSHI TSUSHIN GAKKAI(The Inst. of Electronics and Communication Engineers of Japan), MR 81-11, page 27, 23-9, FIG. 3. FIG. 3 is a figure showing the relationship between the specific surface area of the acicular particles of Co-coated maghemite and the noise level, and as is seen in FIG. 3, the noise level is linearly reduced as the specific surface area of the particle becomes larger. This relationship is also realizable in the acicular ferromagnetic particles and in the acicular ferromagnetic alloy particles.
In order to improve the dispersibility of the magnetic particles in the behicle and the packing thereof in the coating medium, it is required that the magnetic particles to be dispersed in the vehicle have good acicularity, and are uniform in their size, not contaminated by the dendrites and not interwound each other, and as a result, that they have large apparent density.
In order to improve the chroma noise, it is important to improve the surface characteristics of the magnetic recording media, and for that purpose in turn, the magnetic particles are preferably excellent in the dispersibility and the orientability thereof, and in conclusion, it is required that the magnetic particles have good acicularity, and are uniform in their size, not contaminated by dendrites and not interwound each other, and as a result, that they have large apparent density.
These facts are clearly understood from the following description in the afore-mentioned NIKKEI-Electronics on page 85.
"The chroma noise is caused by the roughness with a relatively longer period on the surface of the magnetic tape, and accordingly, it has a close relationship with the technique of coating, and the particles of the better dispersibility and the better orientability are able to give the better surface characteristics to the magnetic tape."
Furthermore, in order to improve the radio frequency output for video signal of the magnetic tape, it is necessary that the coercive force and the residual magnetic flux density (Br) are large enough. For the improvement of the coercive force(Hc) of the magnetic recording media, it is required that the coercive force(Hc) of the magnetic particles themselves is as high as possible.
In order to have a large residual magnetic flux density (Br), the saturation magnetization (.sigma.s) of the magnetic particles should be as large as possible, and Br depends on the dispersibility of the magnetic particles in the behicle and the orientation of the magnetic particles in the coating medium. These facts are clearly recognized from the following descriptions in pages 84 to 85 of the afore-mentioned NIKKEI-Electronics.
"The maximum output of the magnetic tape is decided by the residual magnetic flux density (Br), the coercive force(Hc) and the effective spacing of the magnetic tape. With the increase of Br, the number of magnetic flux entering into the reproducing head increase resulting in the increased output. . . . With the increase of Hc, the self-demagnetization is reduced resulting in the increased output. . . . The first fundamental prerequisite to have a large Br of the magnetic tape is that the saturation magnetization, Is(.sigma.s), possessed by the magnetic particle in a complete state, for instance, in a single crystalline state, is large enough. . . . Br shows changes even in the same material according to the degree of packing thereof and since Br is proportional to the squareness ratio (residual magnetic flux density/saturation magnetic flux density), it is required that the squareness ratio is large enough to have a large Br. . . . It is advantageous for making the squareness ratio larger to use the magnetic particles which are uniform in particle size, high in aspect ratio and excellent in the orientability into the direction of magnetic field. . . ."
As is described in detail as above, in order to fulfill the requirements for the improvement of the quality of the picture, the output, particularly the frequency characteristics and the noise level of the magnetic recording media, that is, the requirement for the highly improved performance of the magnetic recording media, it is necessary to use the magnetic particles provided with the following characteristics, (1) high acicularity, (2) uniform in particle size (3) without being contaminated by dendrites, (4) without having interwound particles, (5) substantially large in apparent density as a result of having a large specific surface area and a highly raised degree of crystallite within the particles and (6) having a high coercive force Hc, and a large saturation magnetization.
In this connection, the magnetic materials which have hitherto been used for preparing the magnetic recording media are the magnetic particles of, for instance, magnetite, maghemite and chromium dioxide having the saturation magnetization, .sigma.s, of 70 to 85 emu/g and the coercive force of 250 to 500 Oe.
Particularly, the fact that saturation magnetization of these magnetic particles is 85 emu/g at the highest in the case where the magnetic particles are oxide and is 70 to 80 emu/g in general is the major cause which limits the reproduced output and the recording density of the magnetic tape.
Furthermore, the magnetic particles containing Co., such as Co-magnetite and Co-maghemite are also used for the same purpose, and although these magnetic particles have a characteristically high coercive force of 400 to 800 Oe, their saturation magnetization is as low as 60 to 80 emu/g.
In recent years, the development of the magnetic particles provided with the characteristics suitable for high output and high recording density, that is, the magnetic particles large in saturation magnetization and high in coercive force has been vigorously carried out, and the thus developed magnetic particles provided with such characteristics are those obtained thermally reducing generally the acicular particles of iron hydroxide, the acicular particles of iron oxide or the acicular particles of iron hydroxide or iron oxide containing metal(s) other than iron in a reducing gas at about 350.degree. C, which are the acicular ferromagnetic particles or acicular ferromagnetic alloy particles.
These magnetic particles characteristically show a remarkably larger saturation magnetization (.sigma.s) and a higher coercive force(Hc) as compared to the hitherto used magnetic particles of iron and the magnetic particles of Co-containing iron oxide, and in the case where they are coated as the magnetic recording media, because of the large residual magnetic flux density(Br) and the high coercive force(Hc) thereof, the thus prepared magnetic tape is suitable for magnetic recording of high density and exhibits a high output and accordingly, such magnetic particles have attracted attention and they have been put to practical use in recent years.
In order that the acicular ferromagnetic particles or acicular ferromagnetic alloy particles exhibit the high coercive force(Hc) and the large saturation magnetization (.sigma.s), it is necessary that (1) they are substantially acicular crystalline in shape, (2) they are uniform in particle size thereof, (3) they are not contaminated by dendrites and (4) they are not interwound with each other, as has been stated, and for producing such magnetic particles provided with the above-mentioned characteristics, and it is necessary that the flocks of Fe(OH.sub.2) which are the precursor of the acicular particles of alpha-FeOOH which are the starting material of the acicular ferromagnetic particles or the acicular ferromagnetic alloy particles are uniform in size and structure and in the same time that the particles of Fe(OH).sub.2 themselves which constitutes the flock are uniform in size and in addition, that the acicular particles of alpha-FeOOH are uniform in particle size and are not contaminated by dendrites.
Such flocks of Fe(OH).sub.2 and acicular particles of alpha-FeOOH are obtained by the respective steps formerly invented and patented by the present inventor(refer to Japanese Patent Publication No. 55-8461 (1980) and Japanese Patent Publication No. 55-32652 (1980)). According to the publications, in the preparation of the acicular particles of alpha-FeOOH by blowing an oxygen-containing gas into an aqueous suspension of Fe(OH).sub.2 of pH of higher than 11 at a temperature of lower than 80.degree. C, which is prepared by admixing an aqueous solution of a ferrous salt with an aqueous solution of an alkali metal hydroxide in amount of more than equivalent to Fe, a water-soluble silicate such as sodium- or potassium silicate is preliminarily added to the aqueous solution of an alkali hydroxide, or to the aqueous suspension of Fe(OH).sub.2 before blowing the oxygen-containing gas thereinto in an amount of 0.1 to 1.7 atomic %(calculated as Si) to Fe in the reaction system.
The thus obtained acicular particles of alpha-FeOOH contain about all amounts of the thus added water-soluble silicate, and the content of Si in the particles is 0.201 to 1.06 atomic % calculated as Si to Fe of the ferrous salt used.
Although the acicular ferromagnetic alloy particles (Si in Fe) obtained by subjecting the thus prepared acicular particles of alpha-FeOOH containing Si to thermal reduction are also uniform in particle size and not contaminated by dendrites and characteristically show a large apparent density, a favorable dispersibility when they are processed into paints, a high packing property in the coating medium prepared thereof and a large residual magnetic flux density(Br), they are small in the specific surface area of 20 m.sup.2 /g at best.
As a result of the present inventor's studies for improving the specific surface area of the thus obtained magnetic particles containing Si, it has been found by the present inventor that in the case of preparing the acicular particles of alpha-FeOOH containing Si and being uniform in particle size without being contaminated by dendrites, the addition of a water-soluble chromium salt into the aqueous solution of a ferrous salt, the aqueous solution of an alkali metal oxide, the aqueous suspension of Fe(OH).sub.2 before blowing an oxygen-containing gas or the aqueous suspension of Fe(OH).sub.2 during oxidation by blowing an oxygen-containing gas thereinto in an amount of 0.1 to 5.0 atomic % calculated as Cr to Fe in the reaction system improves the specific surface area of the acicular ferromagnetic alloy particles containing Cr other than Si obtained by subjecting the thus prepared acicular particles of alpha-FeOOH containing Cr other than Si to thermal reduction.
Further, in the present inventors' studies for improving the coercive force of the acicular ferromagnetic alloy particles, it has been found by the present inventors that in the case of forming the acicular particles of alpha-FeOOH containing Si and Cr, the addition of a water-soluble nickel salt into the aqueous solution of a ferrous salt, the aqueous solution of an alkali metal hydroxide, the aqueous suspension of Fe(OH).sub.2 before blowing an oxygen-containing gas thereinto or the aqueous suspension of Fe(OH).sub.2 during oxidation thereof by blowing the oxygen-containing gas thereinto gives the acicular ferromagnetic alloy particles con taining Ni other than Si and Cr showing a larger coercive force than that of the acicular ferromagnetic alloy particles only containing Si and Cr while retaining the specific surface area as large as that of the acicular ferromagnetic particles only containing Si and Cr.
Furthermore, in the present inventors' studies for further improving the specific surface area and the coercive force of the acicular ferromagnetic alloy particles containing Si, Cr and Ni it has been found that (1) in the case where the amount of addition of the water-soluble chromium salt is over 5.0 atomic %, both the coercive force and the saturation magnetization of the magnetic particles show a tendency of decreasing although the specific surface area is improved, (2) in the case where the amount of addition of the water-soluble nickel salt is over 7.0 atomic %, some foreign matters other than the acicular crystalline are found in the acicular particles of alpha-FeOOH and (3) in the case where in the preparation of the acicular particles of alpha-FeOOH containing Si, Cr and Ni, a water-soluble magnesium is further added preliminarily into the aqueous solution of a ferrous salt, the aqueous solution of an alkali metal hydroxide or the aqueous suspension of Fe(OH).sub.2 before blowing the oxygen-containing gas thereinto or the aqueous suspension of Fe(OH).sub.2 during the blowing of the oxygen-containing gas thereinto for carrying out oxidation in an amount of 0.1 to 15.0 atomic % calculated as Mg to Fe in the reaction system and the thus obtained acicular particles of alpha-FeOOH is thermally reduced, both the specific surface area and the coercive force of the thus obtained acicular ferromagnetic alloy particles containing Si, Cr, Ni and Mg are superior to those of the acicular ferromagnetic alloy particles containing Si, Cr and Ni.